This article presents a unified theory of fiduciary relationships on the basis of the fundamental legal axiom that one may not make a contract with oneself. It characterizes fiduciary law as a law that imposes on whoever places oneself as a fiduciary to another the duty of loyalty -- the duty to act solely for the other’s interests -- as a legal duty enforced by courts. It then shows how fiduciary law practically solves its apparent mix-up between ethics and law by placing the entire burden of disproof on the accused fiduciary, and justifies its disgorgement remedy by the open-ended nature of the duty of loyalty. The article also argues that the primary role of fiduciary law is not to replace but to complement ethics with law.